/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import static com.google.common.collect.CollectPreconditions.checkRemove;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Objects;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.collect.Multiset.Entry;
import com.google.common.math.IntMath;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.Spliterator;
import java.util.stream.Collector;
import javax.annotation.Nullable;

/**
 * Provides static utility methods for creating and working with {@link Multiset} instances.
 *
 * <p>
 * See the Guava User Guide article on
 * <a href= "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#multisets">
 * {@code Multisets}</a>.
 *
 * @author Kevin Bourrillion
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
public final class Multisets {
    private Multisets() {}

    /**
     * Returns a {@code Collector} that accumulates elements into a multiset created via the
     * specified {@code Supplier}, whose elements are the result of applying {@code elementFunction}
     * to the inputs, with counts equal to the result of applying {@code countFunction} to the
     * inputs. Elements are added in encounter order.
     *
     * <p>
     * If the mapped elements contain duplicates (according to {@link Object#equals}), the element
     * will be added more than once, with the count summed over all appearances of the element.
     *
     * <p>
     * Note that {@code stream.collect(toMultiset(function, e -> 1, supplier))} is equivalent to
     * {@code stream.map(function).collect(Collectors.toCollection(supplier))}.
     *
     * @since 22.0
     */
    public static <T, E, M extends Multiset<E>> Collector<T, ?, M> toMultiset(
            java.util.function.Function<T, E> elemFunction, java.util.function.ToIntFunction<T> countFunction,
            java.util.function.Supplier<M> implSupplier) {
        checkNotNull(elemFunction);
        checkNotNull(countFunction);
        checkNotNull(implSupplier);
        return Collector.of(implSupplier, (ms, t) -> ms.add(elemFunction.apply(t), countFunction.applyAsInt(t)),
                (ms1, ms2) -> {
                    ms1.addAll(ms2);
                    return ms1;
                });
    }

    /**
     * Returns an unmodifiable view of the specified multiset. Query operations on the returned
     * multiset "read through" to the specified multiset, and attempts to modify the returned
     * multiset result in an {@link UnsupportedOperationException}.
     *
     * <p>
     * The returned multiset will be serializable if the specified multiset is serializable.
     *
     * @param multiset the multiset for which an unmodifiable view is to be generated
     * @return an unmodifiable view of the multiset
     */
    public static <E> Multiset<E> unmodifiableMultiset(Multiset<? extends E> multiset) {
        if (multiset instanceof UnmodifiableMultiset || multiset instanceof ImmutableMultiset) {
            @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
            Multiset<E> result = (Multiset<E>) multiset;
            return result;
        }
        return new UnmodifiableMultiset<E>(checkNotNull(multiset));
    }

    /**
     * Simply returns its argument.
     *
     * @deprecated no need to use this
     * @since 10.0
     */
    @Deprecated
    public static <E> Multiset<E> unmodifiableMultiset(ImmutableMultiset<E> multiset) {
        return checkNotNull(multiset);
    }

    static class UnmodifiableMultiset<E> extends ForwardingMultiset<E> implements Serializable {
        final Multiset<? extends E> delegate;

        UnmodifiableMultiset(Multiset<? extends E> delegate) {
            this.delegate = delegate;
        }

        @SuppressWarnings("unchecked")
        @Override
        protected Multiset<E> delegate() {
            // This is safe because all non-covariant methods are overriden
            return (Multiset<E>) delegate;
        }

        transient Set<E> elementSet;

        Set<E> createElementSet() {
            return Collections.<E>unmodifiableSet(delegate.elementSet());
        }

        @Override
        public Set<E> elementSet() {
            Set<E> es = elementSet;
            return (es == null) ? elementSet = createElementSet() : es;
        }

        transient Set<Multiset.Entry<E>> entrySet;

        @SuppressWarnings("unchecked")
        @Override
        public Set<Multiset.Entry<E>> entrySet() {
            Set<Multiset.Entry<E>> es = entrySet;
            return (es == null)
                    // Safe because the returned set is made unmodifiable and Entry
                    // itself is readonly
                    ? entrySet = (Set) Collections.unmodifiableSet(delegate.entrySet())
                    : es;
        }

        @Override
        public Iterator<E> iterator() {
            return Iterators.<E>unmodifiableIterator(delegate.iterator());
        }

        @Override
        public boolean add(E element) {
            throw new UnsupportedOperationException();
        }

        @Override
        public int add(E element, int occurences) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean addAll(Collection<? extends E> elementsToAdd) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean remove(Object element) {
            throw new UnsupportedOperationException();
        }

        @Override
        public int remove(Object element, int occurrences) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean removeAll(Collection<?> elementsToRemove) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean retainAll(Collection<?> elementsToRetain) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void clear() {
            throw new UnsupportedOperationException();
        }

        @Override
        public int setCount(E element, int count) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean setCount(E element, int oldCount, int newCount) {
            throw new UnsupportedOperationException();
        }

        private static final long serialVersionUID = 0;
    }

    /**
     * Returns an unmodifiable view of the specified sorted multiset. Query operations on the
     * returned multiset "read through" to the specified multiset, and attempts to modify the
     * returned multiset result in an {@link UnsupportedOperationException}.
     *
     * <p>
     * The returned multiset will be serializable if the specified multiset is serializable.
     *
     * @param sortedMultiset the sorted multiset for which an unmodifiable view is to be generated
     * @return an unmodifiable view of the multiset
     * @since 11.0
     */
    @Beta
    public static <E> SortedMultiset<E> unmodifiableSortedMultiset(SortedMultiset<E> sortedMultiset) {
        // it's in its own file so it can be emulated for GWT
        return new UnmodifiableSortedMultiset<E>(checkNotNull(sortedMultiset));
    }

    /**
     * Returns an immutable multiset entry with the specified element and count. The entry will be
     * serializable if {@code e} is.
     *
     * @param e the element to be associated with the returned entry
     * @param n the count to be associated with the returned entry
     * @throws IllegalArgumentException if {@code n} is negative
     */
    public static <E> Multiset.Entry<E> immutableEntry(@Nullable E e, int n) {
        return new ImmutableEntry<E>(e, n);
    }

    static class ImmutableEntry<E> extends AbstractEntry<E> implements Serializable {
        @Nullable
        private final E element;
        private final int count;

        ImmutableEntry(@Nullable E element, int count) {
            this.element = element;
            this.count = count;
            checkNonnegative(count, "count");
        }

        @Override
        @Nullable
        public final E getElement() {
            return element;
        }

        @Override
        public final int getCount() {
            return count;
        }

        public ImmutableEntry<E> nextInBucket() {
            return null;
        }

        private static final long serialVersionUID = 0;
    }

    /**
     * Returns a view of the elements of {@code unfiltered} that satisfy a predicate. The returned
     * multiset is a live view of {@code unfiltered}; changes to one affect the other.
     *
     * <p>
     * The resulting multiset's iterators, and those of its {@code entrySet()} and
     * {@code elementSet()}, do not support {@code remove()}. However, all other multiset methods
     * supported by {@code unfiltered} are supported by the returned multiset. When given an element
     * that doesn't satisfy the predicate, the multiset's {@code add()} and {@code addAll()} methods
     * throw an {@link IllegalArgumentException}. When methods such as {@code removeAll()} and
     * {@code clear()} are called on the filtered multiset, only elements that satisfy the filter
     * will be removed from the underlying multiset.
     *
     * <p>
     * The returned multiset isn't threadsafe or serializable, even if {@code unfiltered} is.
     *
     * <p>
     * Many of the filtered multiset's methods, such as {@code size()}, iterate across every element
     * in the underlying multiset and determine which elements satisfy the filter. When a live view
     * is <i>not</i> needed, it may be faster to copy the returned multiset and use the copy.
     *
     * <p>
     * <b>Warning:</b> {@code predicate} must be <i>consistent with equals</i>, as documented at
     * {@link Predicate#apply}. Do not provide a predicate such as
     * {@code Predicates.instanceOf(ArrayList.class)}, which is inconsistent with equals. (See
     * {@link Iterables#filter(Iterable, Class)} for related functionality.)
     *
     * @since 14.0
     */
    @Beta
    public static <E> Multiset<E> filter(Multiset<E> unfiltered, Predicate<? super E> predicate) {
        if (unfiltered instanceof FilteredMultiset) {
            // Support clear(), removeAll(), and retainAll() when filtering a filtered
            // collection.
            FilteredMultiset<E> filtered = (FilteredMultiset<E>) unfiltered;
            Predicate<E> combinedPredicate = Predicates.<E>and(filtered.predicate, predicate);
            return new FilteredMultiset<E>(filtered.unfiltered, combinedPredicate);
        }
        return new FilteredMultiset<E>(unfiltered, predicate);
    }

    private static final class FilteredMultiset<E> extends AbstractMultiset<E> {
        final Multiset<E> unfiltered;
        final Predicate<? super E> predicate;

        FilteredMultiset(Multiset<E> unfiltered, Predicate<? super E> predicate) {
            this.unfiltered = checkNotNull(unfiltered);
            this.predicate = checkNotNull(predicate);
        }

        @Override
        public UnmodifiableIterator<E> iterator() {
            return Iterators.filter(unfiltered.iterator(), predicate);
        }

        @Override
        Set<E> createElementSet() {
            return Sets.filter(unfiltered.elementSet(), predicate);
        }

        @Override
        Set<Entry<E>> createEntrySet() {
            return Sets.filter(unfiltered.entrySet(), new Predicate<Entry<E>>() {
                @Override
                public boolean apply(Entry<E> entry) {
                    return predicate.apply(entry.getElement());
                }
            });
        }

        @Override
        Iterator<Entry<E>> entryIterator() {
            throw new AssertionError("should never be called");
        }

        @Override
        int distinctElements() {
            return elementSet().size();
        }

        @Override
        public int count(@Nullable Object element) {
            int count = unfiltered.count(element);
            if (count > 0) {
                @SuppressWarnings("unchecked") // element is equal to an E
                E e = (E) element;
                return predicate.apply(e) ? count : 0;
            }
            return 0;
        }

        @Override
        public int add(@Nullable E element, int occurrences) {
            checkArgument(predicate.apply(element), "Element %s does not match predicate %s", element, predicate);
            return unfiltered.add(element, occurrences);
        }

        @Override
        public int remove(@Nullable Object element, int occurrences) {
            checkNonnegative(occurrences, "occurrences");
            if (occurrences == 0) {
                return count(element);
            } else {
                return contains(element) ? unfiltered.remove(element, occurrences) : 0;
            }
        }

        @Override
        public void clear() {
            elementSet().clear();
        }
    }

    /**
     * Returns the expected number of distinct elements given the specified elements. The number of
     * distinct elements is only computed if {@code
     * elements} is an instance of {@code Multiset}; otherwise the default value of 11 is returned.
     */
    static int inferDistinctElements(Iterable<?> elements) {
        if (elements instanceof Multiset) {
            return ((Multiset<?>) elements).elementSet().size();
        }
        return 11; // initial capacity will be rounded up to 16
    }

    /**
     * Returns an unmodifiable view of the union of two multisets. In the returned multiset, the
     * count of each element is the <i>maximum</i> of its counts in the two backing multisets. The
     * iteration order of the returned multiset matches that of the element set of {@code multiset1}
     * followed by the members of the element set of {@code multiset2} that are not contained in
     * {@code multiset1}, with repeated occurrences of the same element appearing consecutively.
     *
     * <p>
     * Results are undefined if {@code multiset1} and {@code multiset2} are based on different
     * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset} are).
     *
     * @since 14.0
     */
    @Beta
    public static <E> Multiset<E> union(final Multiset<? extends E> multiset1, final Multiset<? extends E> multiset2) {
        checkNotNull(multiset1);
        checkNotNull(multiset2);

        return new AbstractMultiset<E>() {
            @Override
            public boolean contains(@Nullable Object element) {
                return multiset1.contains(element) || multiset2.contains(element);
            }

            @Override
            public boolean isEmpty() {
                return multiset1.isEmpty() && multiset2.isEmpty();
            }

            @Override
            public int count(Object element) {
                return Math.max(multiset1.count(element), multiset2.count(element));
            }

            @Override
            Set<E> createElementSet() {
                return Sets.union(multiset1.elementSet(), multiset2.elementSet());
            }

            @Override
            Iterator<Entry<E>> entryIterator() {
                final Iterator<? extends Entry<? extends E>> iterator1 = multiset1.entrySet().iterator();
                final Iterator<? extends Entry<? extends E>> iterator2 = multiset2.entrySet().iterator();
                // TODO(lowasser): consider making the entries live views
                return new AbstractIterator<Entry<E>>() {
                    @Override
                    protected Entry<E> computeNext() {
                        if (iterator1.hasNext()) {
                            Entry<? extends E> entry1 = iterator1.next();
                            E element = entry1.getElement();
                            int count = Math.max(entry1.getCount(), multiset2.count(element));
                            return immutableEntry(element, count);
                        }
                        while (iterator2.hasNext()) {
                            Entry<? extends E> entry2 = iterator2.next();
                            E element = entry2.getElement();
                            if (!multiset1.contains(element)) {
                                return immutableEntry(element, entry2.getCount());
                            }
                        }
                        return endOfData();
                    }
                };
            }

            @Override
            int distinctElements() {
                return elementSet().size();
            }
        };
    }

    /**
     * Returns an unmodifiable view of the intersection of two multisets. In the returned multiset,
     * the count of each element is the <i>minimum</i> of its counts in the two backing multisets,
     * with elements that would have a count of 0 not included. The iteration order of the returned
     * multiset matches that of the element set of {@code multiset1}, with repeated occurrences of
     * the same element appearing consecutively.
     *
     * <p>
     * Results are undefined if {@code multiset1} and {@code multiset2} are based on different
     * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset} are).
     *
     * @since 2.0
     */
    public static <E> Multiset<E> intersection(final Multiset<E> multiset1, final Multiset<?> multiset2) {
        checkNotNull(multiset1);
        checkNotNull(multiset2);

        return new AbstractMultiset<E>() {
            @Override
            public int count(Object element) {
                int count1 = multiset1.count(element);
                return (count1 == 0) ? 0 : Math.min(count1, multiset2.count(element));
            }

            @Override
            Set<E> createElementSet() {
                return Sets.intersection(multiset1.elementSet(), multiset2.elementSet());
            }

            @Override
            Iterator<Entry<E>> entryIterator() {
                final Iterator<Entry<E>> iterator1 = multiset1.entrySet().iterator();
                // TODO(lowasser): consider making the entries live views
                return new AbstractIterator<Entry<E>>() {
                    @Override
                    protected Entry<E> computeNext() {
                        while (iterator1.hasNext()) {
                            Entry<E> entry1 = iterator1.next();
                            E element = entry1.getElement();
                            int count = Math.min(entry1.getCount(), multiset2.count(element));
                            if (count > 0) {
                                return immutableEntry(element, count);
                            }
                        }
                        return endOfData();
                    }
                };
            }

            @Override
            int distinctElements() {
                return elementSet().size();
            }
        };
    }

    /**
     * Returns an unmodifiable view of the sum of two multisets. In the returned multiset, the count
     * of each element is the <i>sum</i> of its counts in the two backing multisets. The iteration
     * order of the returned multiset matches that of the element set of {@code multiset1} followed
     * by the members of the element set of {@code multiset2} that are not contained in
     * {@code multiset1}, with repeated occurrences of the same element appearing consecutively.
     *
     * <p>
     * Results are undefined if {@code multiset1} and {@code multiset2} are based on different
     * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset} are).
     *
     * @since 14.0
     */
    @Beta
    public static <E> Multiset<E> sum(final Multiset<? extends E> multiset1, final Multiset<? extends E> multiset2) {
        checkNotNull(multiset1);
        checkNotNull(multiset2);

        // TODO(lowasser): consider making the entries live views
        return new AbstractMultiset<E>() {
            @Override
            public boolean contains(@Nullable Object element) {
                return multiset1.contains(element) || multiset2.contains(element);
            }

            @Override
            public boolean isEmpty() {
                return multiset1.isEmpty() && multiset2.isEmpty();
            }

            @Override
            public int size() {
                return IntMath.saturatedAdd(multiset1.size(), multiset2.size());
            }

            @Override
            public int count(Object element) {
                return multiset1.count(element) + multiset2.count(element);
            }

            @Override
            Set<E> createElementSet() {
                return Sets.union(multiset1.elementSet(), multiset2.elementSet());
            }

            @Override
            Iterator<Entry<E>> entryIterator() {
                final Iterator<? extends Entry<? extends E>> iterator1 = multiset1.entrySet().iterator();
                final Iterator<? extends Entry<? extends E>> iterator2 = multiset2.entrySet().iterator();
                return new AbstractIterator<Entry<E>>() {
                    @Override
                    protected Entry<E> computeNext() {
                        if (iterator1.hasNext()) {
                            Entry<? extends E> entry1 = iterator1.next();
                            E element = entry1.getElement();
                            int count = entry1.getCount() + multiset2.count(element);
                            return immutableEntry(element, count);
                        }
                        while (iterator2.hasNext()) {
                            Entry<? extends E> entry2 = iterator2.next();
                            E element = entry2.getElement();
                            if (!multiset1.contains(element)) {
                                return immutableEntry(element, entry2.getCount());
                            }
                        }
                        return endOfData();
                    }
                };
            }

            @Override
            int distinctElements() {
                return elementSet().size();
            }
        };
    }

    /**
     * Returns an unmodifiable view of the difference of two multisets. In the returned multiset,
     * the count of each element is the result of the <i>zero-truncated subtraction</i> of its count
     * in the second multiset from its count in the first multiset, with elements that would have a
     * count of 0 not included. The iteration order of the returned multiset matches that of the
     * element set of {@code multiset1}, with repeated occurrences of the same element appearing
     * consecutively.
     *
     * <p>
     * Results are undefined if {@code multiset1} and {@code multiset2} are based on different
     * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset} are).
     *
     * @since 14.0
     */
    @Beta
    public static <E> Multiset<E> difference(final Multiset<E> multiset1, final Multiset<?> multiset2) {
        checkNotNull(multiset1);
        checkNotNull(multiset2);

        // TODO(lowasser): consider making the entries live views
        return new AbstractMultiset<E>() {
            @Override
            public int count(@Nullable Object element) {
                int count1 = multiset1.count(element);
                return (count1 == 0) ? 0 : Math.max(0, count1 - multiset2.count(element));
            }

            @Override
            Iterator<Entry<E>> entryIterator() {
                final Iterator<Entry<E>> iterator1 = multiset1.entrySet().iterator();
                return new AbstractIterator<Entry<E>>() {
                    @Override
                    protected Entry<E> computeNext() {
                        while (iterator1.hasNext()) {
                            Entry<E> entry1 = iterator1.next();
                            E element = entry1.getElement();
                            int count = entry1.getCount() - multiset2.count(element);
                            if (count > 0) {
                                return immutableEntry(element, count);
                            }
                        }
                        return endOfData();
                    }
                };
            }

            @Override
            int distinctElements() {
                return Iterators.size(entryIterator());
            }
        };
    }

    /**
     * Returns {@code true} if {@code subMultiset.count(o) <=
     * superMultiset.count(o)} for all {@code o}.
     *
     * @since 10.0
     */
    @CanIgnoreReturnValue
    public static boolean containsOccurrences(Multiset<?> superMultiset, Multiset<?> subMultiset) {
        checkNotNull(superMultiset);
        checkNotNull(subMultiset);
        for (Entry<?> entry : subMultiset.entrySet()) {
            int superCount = superMultiset.count(entry.getElement());
            if (superCount < entry.getCount()) {
                return false;
            }
        }
        return true;
    }

    /**
     * Modifies {@code multisetToModify} so that its count for an element {@code e} is at most
     * {@code multisetToRetain.count(e)}.
     *
     * <p>
     * To be precise, {@code multisetToModify.count(e)} is set to
     * {@code Math.min(multisetToModify.count(e),
     * multisetToRetain.count(e))}. This is similar to {@link #intersection(Multiset, Multiset)
     * intersection} {@code (multisetToModify, multisetToRetain)}, but mutates
     * {@code multisetToModify} instead of returning a view.
     *
     * <p>
     * In contrast, {@code multisetToModify.retainAll(multisetToRetain)} keeps all occurrences of
     * elements that appear at all in {@code
     * multisetToRetain}, and deletes all occurrences of all other elements.
     *
     * @return {@code true} if {@code multisetToModify} was changed as a result of this operation
     * @since 10.0
     */
    @CanIgnoreReturnValue
    public static boolean retainOccurrences(Multiset<?> multisetToModify, Multiset<?> multisetToRetain) {
        return retainOccurrencesImpl(multisetToModify, multisetToRetain);
    }

    /**
     * Delegate implementation which cares about the element type.
     */
    private static <E> boolean retainOccurrencesImpl(Multiset<E> multisetToModify, Multiset<?> occurrencesToRetain) {
        checkNotNull(multisetToModify);
        checkNotNull(occurrencesToRetain);
        // Avoiding ConcurrentModificationExceptions is tricky.
        Iterator<Entry<E>> entryIterator = multisetToModify.entrySet().iterator();
        boolean changed = false;
        while (entryIterator.hasNext()) {
            Entry<E> entry = entryIterator.next();
            int retainCount = occurrencesToRetain.count(entry.getElement());
            if (retainCount == 0) {
                entryIterator.remove();
                changed = true;
            } else if (retainCount < entry.getCount()) {
                multisetToModify.setCount(entry.getElement(), retainCount);
                changed = true;
            }
        }
        return changed;
    }

    /**
     * For each occurrence of an element {@code e} in {@code occurrencesToRemove}, removes one
     * occurrence of {@code e} in {@code multisetToModify}.
     *
     * <p>
     * Equivalently, this method modifies {@code multisetToModify} so that
     * {@code multisetToModify.count(e)} is set to {@code Math.max(0, multisetToModify.count(e) -
     * Iterables.frequency(occurrencesToRemove, e))}.
     *
     * <p>
     * This is <i>not</i> the same as {@code multisetToModify.} {@link Multiset#removeAll
     * removeAll}{@code (occurrencesToRemove)}, which removes all occurrences of elements that
     * appear in {@code occurrencesToRemove}. However, this operation <i>is</i> equivalent to,
     * albeit sometimes more efficient than, the following:
     * 
     * <pre>
     *    {@code
     *
     *   for (E e : occurrencesToRemove) {
     *     multisetToModify.remove(e);
     *   }}
     * </pre>
     *
     * @return {@code true} if {@code multisetToModify} was changed as a result of this operation
     * @since 18.0 (present in 10.0 with a requirement that the second parameter be a
     *        {@code Multiset})
     */
    @CanIgnoreReturnValue
    public static boolean removeOccurrences(Multiset<?> multisetToModify, Iterable<?> occurrencesToRemove) {
        if (occurrencesToRemove instanceof Multiset) {
            return removeOccurrences(multisetToModify, (Multiset<?>) occurrencesToRemove);
        } else {
            checkNotNull(multisetToModify);
            checkNotNull(occurrencesToRemove);
            boolean changed = false;
            for (Object o : occurrencesToRemove) {
                changed |= multisetToModify.remove(o);
            }
            return changed;
        }
    }

    /**
     * For each occurrence of an element {@code e} in {@code occurrencesToRemove}, removes one
     * occurrence of {@code e} in {@code multisetToModify}.
     *
     * <p>
     * Equivalently, this method modifies {@code multisetToModify} so that
     * {@code multisetToModify.count(e)} is set to {@code Math.max(0, multisetToModify.count(e) -
     * occurrencesToRemove.count(e))}.
     *
     * <p>
     * This is <i>not</i> the same as {@code multisetToModify.} {@link Multiset#removeAll
     * removeAll}{@code (occurrencesToRemove)}, which removes all occurrences of elements that
     * appear in {@code occurrencesToRemove}. However, this operation <i>is</i> equivalent to,
     * albeit sometimes more efficient than, the following:
     * 
     * <pre>
     *    {@code
     *
     *   for (E e : occurrencesToRemove) {
     *     multisetToModify.remove(e);
     *   }}
     * </pre>
     *
     * @return {@code true} if {@code multisetToModify} was changed as a result of this operation
     * @since 10.0 (missing in 18.0 when only the overload taking an {@code Iterable} was present)
     */
    @CanIgnoreReturnValue
    public static boolean removeOccurrences(Multiset<?> multisetToModify, Multiset<?> occurrencesToRemove) {
        checkNotNull(multisetToModify);
        checkNotNull(occurrencesToRemove);

        boolean changed = false;
        Iterator<? extends Entry<?>> entryIterator = multisetToModify.entrySet().iterator();
        while (entryIterator.hasNext()) {
            Entry<?> entry = entryIterator.next();
            int removeCount = occurrencesToRemove.count(entry.getElement());
            if (removeCount >= entry.getCount()) {
                entryIterator.remove();
                changed = true;
            } else if (removeCount > 0) {
                multisetToModify.remove(entry.getElement(), removeCount);
                changed = true;
            }
        }
        return changed;
    }

    /**
     * Implementation of the {@code equals}, {@code hashCode}, and {@code toString} methods of
     * {@link Multiset.Entry}.
     */
    abstract static class AbstractEntry<E> implements Multiset.Entry<E> {
        /**
         * Indicates whether an object equals this entry, following the behavior specified in
         * {@link Multiset.Entry#equals}.
         */
        @Override
        public boolean equals(@Nullable Object object) {
            if (object instanceof Multiset.Entry) {
                Multiset.Entry<?> that = (Multiset.Entry<?>) object;
                return this.getCount() == that.getCount() && Objects.equal(this.getElement(), that.getElement());
            }
            return false;
        }

        /**
         * Return this entry's hash code, following the behavior specified in
         * {@link Multiset.Entry#hashCode}.
         */
        @Override
        public int hashCode() {
            E e = getElement();
            return ((e == null) ? 0 : e.hashCode()) ^ getCount();
        }

        /**
         * Returns a string representation of this multiset entry. The string representation
         * consists of the associated element if the associated count is one, and otherwise the
         * associated element followed by the characters " x " (space, x and space) followed by the
         * count. Elements and counts are converted to strings as by {@code String.valueOf}.
         */
        @Override
        public String toString() {
            String text = String.valueOf(getElement());
            int n = getCount();
            return (n == 1) ? text : (text + " x " + n);
        }
    }

    /**
     * An implementation of {@link Multiset#equals}.
     */
    static boolean equalsImpl(Multiset<?> multiset, @Nullable Object object) {
        if (object == multiset) {
            return true;
        }
        if (object instanceof Multiset) {
            Multiset<?> that = (Multiset<?>) object;
            /*
             * We can't simply check whether the entry sets are equal, since that approach fails
             * when a TreeMultiset has a comparator that returns 0 when passed unequal elements.
             */

            if (multiset.size() != that.size() || multiset.entrySet().size() != that.entrySet().size()) {
                return false;
            }
            for (Entry<?> entry : that.entrySet()) {
                if (multiset.count(entry.getElement()) != entry.getCount()) {
                    return false;
                }
            }
            return true;
        }
        return false;
    }

    /**
     * An implementation of {@link Multiset#addAll}.
     */
    static <E> boolean addAllImpl(Multiset<E> self, Collection<? extends E> elements) {
        if (elements.isEmpty()) {
            return false;
        }
        if (elements instanceof Multiset) {
            Multiset<? extends E> that = cast(elements);
            for (Entry<? extends E> entry : that.entrySet()) {
                self.add(entry.getElement(), entry.getCount());
            }
        } else {
            Iterators.addAll(self, elements.iterator());
        }
        return true;
    }

    /**
     * An implementation of {@link Multiset#removeAll}.
     */
    static boolean removeAllImpl(Multiset<?> self, Collection<?> elementsToRemove) {
        Collection<?> collection =
                (elementsToRemove instanceof Multiset) ? ((Multiset<?>) elementsToRemove).elementSet()
                        : elementsToRemove;

        return self.elementSet().removeAll(collection);
    }

    /**
     * An implementation of {@link Multiset#retainAll}.
     */
    static boolean retainAllImpl(Multiset<?> self, Collection<?> elementsToRetain) {
        checkNotNull(elementsToRetain);
        Collection<?> collection =
                (elementsToRetain instanceof Multiset) ? ((Multiset<?>) elementsToRetain).elementSet()
                        : elementsToRetain;

        return self.elementSet().retainAll(collection);
    }

    /**
     * An implementation of {@link Multiset#setCount(Object, int)}.
     */
    static <E> int setCountImpl(Multiset<E> self, E element, int count) {
        checkNonnegative(count, "count");

        int oldCount = self.count(element);

        int delta = count - oldCount;
        if (delta > 0) {
            self.add(element, delta);
        } else if (delta < 0) {
            self.remove(element, -delta);
        }

        return oldCount;
    }

    /**
     * An implementation of {@link Multiset#setCount(Object, int, int)}.
     */
    static <E> boolean setCountImpl(Multiset<E> self, E element, int oldCount, int newCount) {
        checkNonnegative(oldCount, "oldCount");
        checkNonnegative(newCount, "newCount");

        if (self.count(element) == oldCount) {
            self.setCount(element, newCount);
            return true;
        } else {
            return false;
        }
    }

    abstract static class ElementSet<E> extends Sets.ImprovedAbstractSet<E> {
        abstract Multiset<E> multiset();

        @Override
        public void clear() {
            multiset().clear();
        }

        @Override
        public boolean contains(Object o) {
            return multiset().contains(o);
        }

        @Override
        public boolean containsAll(Collection<?> c) {
            return multiset().containsAll(c);
        }

        @Override
        public boolean isEmpty() {
            return multiset().isEmpty();
        }

        @Override
        public Iterator<E> iterator() {
            return new TransformedIterator<Entry<E>, E>(multiset().entrySet().iterator()) {
                @Override
                E transform(Entry<E> entry) {
                    return entry.getElement();
                }
            };
        }

        @Override
        public boolean remove(Object o) {
            return multiset().remove(o, Integer.MAX_VALUE) > 0;
        }

        @Override
        public int size() {
            return multiset().entrySet().size();
        }
    }

    abstract static class EntrySet<E> extends Sets.ImprovedAbstractSet<Entry<E>> {
        abstract Multiset<E> multiset();

        @Override
        public boolean contains(@Nullable Object o) {
            if (o instanceof Entry) {
                /*
                 * The GWT compiler wrongly issues a warning here.
                 */
                @SuppressWarnings("cast")
                Entry<?> entry = (Entry<?>) o;
                if (entry.getCount() <= 0) {
                    return false;
                }
                int count = multiset().count(entry.getElement());
                return count == entry.getCount();
            }
            return false;
        }

        // GWT compiler warning; see contains().
        @SuppressWarnings("cast")
        @Override
        public boolean remove(Object object) {
            if (object instanceof Multiset.Entry) {
                Entry<?> entry = (Entry<?>) object;
                Object element = entry.getElement();
                int entryCount = entry.getCount();
                if (entryCount != 0) {
                    // Safe as long as we never add a new entry, which we won't.
                    @SuppressWarnings("unchecked")
                    Multiset<Object> multiset = (Multiset) multiset();
                    return multiset.setCount(element, entryCount, 0);
                }
            }
            return false;
        }

        @Override
        public void clear() {
            multiset().clear();
        }
    }

    /**
     * An implementation of {@link Multiset#iterator}.
     */
    static <E> Iterator<E> iteratorImpl(Multiset<E> multiset) {
        return new MultisetIteratorImpl<E>(multiset, multiset.entrySet().iterator());
    }

    static final class MultisetIteratorImpl<E> implements Iterator<E> {
        private final Multiset<E> multiset;
        private final Iterator<Entry<E>> entryIterator;
        private Entry<E> currentEntry;

        /** Count of subsequent elements equal to current element */
        private int laterCount;

        /** Count of all elements equal to current element */
        private int totalCount;

        private boolean canRemove;

        MultisetIteratorImpl(Multiset<E> multiset, Iterator<Entry<E>> entryIterator) {
            this.multiset = multiset;
            this.entryIterator = entryIterator;
        }

        @Override
        public boolean hasNext() {
            return laterCount > 0 || entryIterator.hasNext();
        }

        @Override
        public E next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            if (laterCount == 0) {
                currentEntry = entryIterator.next();
                totalCount = laterCount = currentEntry.getCount();
            }
            laterCount--;
            canRemove = true;
            return currentEntry.getElement();
        }

        @Override
        public void remove() {
            checkRemove(canRemove);
            if (totalCount == 1) {
                entryIterator.remove();
            } else {
                multiset.remove(currentEntry.getElement());
            }
            totalCount--;
            canRemove = false;
        }
    }

    static <E> Spliterator<E> spliteratorImpl(Multiset<E> multiset) {
        Spliterator<Entry<E>> entrySpliterator = multiset.entrySet().spliterator();
        return CollectSpliterators
                .flatMap(entrySpliterator,
                        entry -> Collections.nCopies(entry.getCount(), entry.getElement()).spliterator(),
                        Spliterator.SIZED | (entrySpliterator.characteristics()
                                & (Spliterator.ORDERED | Spliterator.NONNULL | Spliterator.IMMUTABLE)),
                        multiset.size());
    }

    /**
     * An implementation of {@link Multiset#size}.
     */
    static int sizeImpl(Multiset<?> multiset) {
        long size = 0;
        for (Entry<?> entry : multiset.entrySet()) {
            size += entry.getCount();
        }
        return Ints.saturatedCast(size);
    }

    /**
     * Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557
     */
    static <T> Multiset<T> cast(Iterable<T> iterable) {
        return (Multiset<T>) iterable;
    }

    private static final Ordering<Entry<?>> DECREASING_COUNT_ORDERING = new Ordering<Entry<?>>() {
        @Override
        public int compare(Entry<?> entry1, Entry<?> entry2) {
            return Ints.compare(entry2.getCount(), entry1.getCount());
        }
    };

    /**
     * Returns a copy of {@code multiset} as an {@link ImmutableMultiset} whose iteration order is
     * highest count first, with ties broken by the iteration order of the original multiset.
     *
     * @since 11.0
     */
    @Beta
    public static <E> ImmutableMultiset<E> copyHighestCountFirst(Multiset<E> multiset) {
        List<Entry<E>> sortedEntries = Multisets.DECREASING_COUNT_ORDERING.immutableSortedCopy(multiset.entrySet());
        return ImmutableMultiset.copyFromEntries(sortedEntries);
    }
}
